As a result of the highly fluctuating infeed of renewable energy (sun/wind) into power grids, the dilemma arises with regard to CHP plants (CHP=combined heat and power) that they are supposed on the one hand to supply thermal energy in accordance with district heating requirements and on the other hand to supply electricity in accordance with grid requirements, wherein it may become uneconomic for example at night or at weekends to burn fossil fuels to generate district heating because the price of electricity is then often very low (in future possibly even ever more frequently negative). To decouple the production of thermal and electrical energy, district heat storage devices are therefore increasingly being used, preferably in the form of pressureless stratified hot water storage devices. Since pressureless stratified storage devices can only be charged at water temperatures <100° C. (in practice this is mostly limited to <95° C.) and since heat exchangers with additional temperature differences are often interposed between storage device and district heating network (since the district heating network often has a significantly higher operating pressure than the buffer tank), the actual useful temperature in discharge mode sometimes falls below 85° C. In transitional periods (spring/fall) and above all in winter, however, the conventional district heating networks are operated at flow temperatures markedly above 95° C. (up to around 135/140° C.): apart from in the summer when district heating flow temperatures are lower than 85-95° C., it is therefore not possible with the concept that is currently conventional to operate the district heating network solely from the heat storage device alone and to completely shut down the CHP plant or the fired or electrical auxiliary boiler.
To be able to charge stratified hot water storage devices at higher district heat flow temperatures over 95° C., sometimes pressure vessels are used which can be charged and discharged at correspondingly higher flow temperatures due to the pressure-dependent saturation temperatures. The major disadvantage of pressure vessels is however significantly higher testing overheads during production, and recurrent testing (for example pressure testing with cold water after 10 years, recurrent visual inspection of all weld seams, for which purpose all insulation would have to be removed from the tanks). In addition, insurance premiums rise steeply for pressure vessels depending on the pressure and volume (since the risk has been assessed as very high by the insurers) and thus pressure vessels are at present very seldom used for large storage volumes for cost reasons.